1. Field of the Invention
The present invention relates to an ATM switching apparatus and an ATM communications network, which offers SVC (Switched Virtual Connection) services.
2. Description of the Related Art
Services offered in a B-ISDN (Broadband-Integrated Service Digital Network) constructed of ATM switching apparatuses are classified into connection-oriented services and connectionless services. The connection oriented services thereof are subdivided into a PVC (Permanent Virtual Connection) service in which a calling user and a bandwidth are semi-permanent, and an SVC (Switched Virtual Connection) service in which the communication connection is dynamically established according to the called user, the bandwidth and a quality of service that are declared from a calling user.
Standardization of signalling protocols at a user-network interface (UNI) and network-node interface (NNI) for actualizing the SVC service, is advanced stepwise (the way in which the functions are extended) by the ITU-T (International telecommunication Union-Telecommunication Standardization Sector) as a main organization. The signaling protocols at the UNI and NNI for B-ISDN are respectively known as DSS2 (Digital Signalling System No. 2) and B-ISUP (B-ISDN User Part).
Hereinafter, an outline of the DSS2 and B-ISUP will be explained. First, formats of DSS2, B-ISUP messages are explained.
DSS2 message, as shown in FIG. 11, is composed of a protocol discriminator, a call reference length, a call reference, a message type, a message length, and some information elements.
The protocol discriminator is information identifying the specific protocol in which the message are to be interpreted, and is set to "00001001" for the DSS2 message. The call reference length is information specifying the number of octets used for the call reference. The call reference is information identifying the connection to which the message pertains. Note that the call reference identifies the connection at the local UNI only, but has no end-to-end significance. That is, a connection is identified by two independent call references valid at the near-end and far-end UNIs.
The message type is information identifying the specific type of message, such as SETUP, CONNECT, and so on. The message length is information specifying the number of octets used for the information elements, and is set to 0000H when the message contains no information element. All DSS2 messages must contain the protocol discriminator, the call reference length, the call reference, the message type and the message length, and so they are at least nine octets long.
Each of the information elements prepared for DSS2 messages consists of an information element identifier, a coding standard, an information element instruction field, an information element content length and information element content.
The element identifier is information identifying the information element. The coding standard is information identifying the regulation with which the information element are coded, and is usually set to "00" for ITU-T coding. The information instruction field contains an action indicator indicating the receiving node how to act (clear call, discard and proceed, and so on) when the information element cannot be interpreted, and a flag bit indicating whether or not the action indicator is to be followed. The information element content length is information specifying the number of octets used for the information element content.
The set of DSS2 messages is divided into call establishment messages, call clearing messages, messages used during the information phase of the connection, and a set of miscellaneous messages. Each DSS2 message may contain multiple information elements, some are mandatory and some are optional, depending on the type and its use.
B-ISUP message, as shown in FIG. 12, is composed of MTP (Message Transfer Protocol)-3 header, a message type, a message length, a message compatibility information and some parameters.
The MTP-3 header is composed of a service information octet (SIO) indicating the specific protocol in which the message are to be interpreted, and a routing label. The routing label is used to route the message from one switching apparatus to another. It is composed of a destination point code (DPS), an originating point code (OPS), and a signalling link selection number (SLS). The DPS is the address of the switching apparatus to which the message must be delivered and the OPS is the address of the switching apparatus which originates the message. The SLS defines a virtual routes between the switching apparatuses. It is used assign the messages belonging to different transactions to separate routes.
The message type is information to identify the particular function carried out by the message. The message length specifies the number of octets used for the message compatibility information and the parameters. The message compatibility information indicates the receiving node how to act (disconnect call, ignore this message, and so on) in case that the message cannot be interpreted.
Each of the parameters prepared for B-ISUP messages is composed of a parameter name, a parameter length, parameter compatibility information and parameter contents. The parameter name is information identifying the parameter. The parameter length is information specifying the number of octets used for the parameter compatibility information and the parameter content. The parameter compatibility information is information indicating a node, which receives this parameter, how to act in case that the parameter cannot be interpreted.
The DSS2 and the B-ISUP are separate protocols, however, their messages, information elements and parameters are closely associated with each other.
For example, as shown in FIG. 13, an IAM (Initial address message), which is a B-ISUP message corresponding to the DSS2 SETUP message, may contain parameters corresponding to information elements used in the SETUP message. As shown in FIG. 14, an ANM (Answer message), which is a B-ISUP message corresponding to the DSS2 CONNECT message, may contain parameters corresponding to information elements used in the CONNECT message.
Next, an outline of a call set up operation using the DSS2 and B-ISUP will be explained by exemplifying a case of setting a call between two terminals (calling and called terminals) via two switching apparatuses (originating and destination switching apparatuses).
The call set up operation starts when the calling terminal sends to the originating switching apparatus (switching apparatus accommodating the calling terminal itself) the SETUP message containing pieces of information elements such as a called party number information element, an ATM traffic descriptor information element, a broadband bearer capability information element, a QOS (quality of service) parameter information element and so on.
The ATM traffic descriptor information element, as shown in FIG. 15, contains set of traffic parameters. The broadband bearer capability information element, as shown in FIG. 16, contains information indicating the broadband service to be offered. Note that the broadband bearer capability information element shown in FIG. 16 is that defined in the recommendation Q.2931. In the new recommendation Q.2931.2, it is defined that octet 5aof the broadband bearer capability information element is used as an ATM transfer capability field as shown in FIG. 17.
Upon receiving the SETUP message, the originating switching apparatus recognizes a destination switching apparatus from the called party number contained in the SETUP message. Then, it recognizes the virtual path connections (VPCs) provided between the destination switching apparatus and the originating switching apparatus itself.
Subsequently, it calculates a bandwidth value to be reserved for the call (connection) requested on the basis of the ATM traffic descriptor information element, a broadband bearer capability information element, a QOS (quality of service) parameter information element and so on (hereinafter referred to as bandwidth negotiation data) contained in the SETUP message. Using the calculated bandwidth value and a using condition (a free bandwidth value) of each VPC, the originating switching apparatus selects one VPC which can be used for the requested connection. Note that this selection is made by specifying the VPC whose free bandwidth value is over the calculated bandwidth value and has a minimum difference from the calculated bandwidth value.
After selecting the VPC, the originating switching apparatus sends to the destination switching apparatus the IAM containing a connection element identifier parameter in which the identifier of the selected VPC, namely VPCI, is set along with a variety of parameters corresponding to the information elements included in the SETUP message through a VC (Virtual Channel) for signalling.
The destination switching apparatus receiving the IAM at first calculates a bandwidth to be reserved for the requested connection using the bandwidth negotiation data contained in the IAM. Then, it judges whether the IAM should be accepted on the basis of the calculated bandwidth value and the using condition of the VPC whose VPCI is set in the IAM.
If accepted, the destination switching apparatus sends an IAA (IAM Acknowledgement message), format thereof is shown in FIG. 18, indicating that the IAM has been accepted to the originating switching apparatus. Thereafter, if the called terminal is in a free status, the destination switching apparatus sends a SETUP message to the called terminal and sends an address completion message ACM.
The called terminal receiving the SETUP message returns to the destination switching apparatus and returns a CONNECT message a CALL PROC message indicating that a call establishment has began.
The destination switching apparatus receiving the CALL PROC message sends to the originating switching apparatus a call progress message CPG indicating that call is being processed without problems. The originating switching apparatus receiving the CPG sends an ALERTING message to the calling terminal.
Moreover, the destination switching apparatus, when receiving the CONNECT message, sends an answer message ANM indicating that the called terminal has answered the call to the originating switching apparatus. The originating switching apparatus receiving the ANM sends a CONNECT message to the calling terminal, whereby the calling and called terminals are brought into a communicable status.
On the other hand, when the destination switching apparatus judges that the IAM is not acceptable, it sends an IAR (IAM Reject message), format thereof is shown in FIG. 19, indicating that the IAM is refused due to the unavailability of resources to the originating switching apparatus.
When receiving the IAR, the originating switching apparatus selects another VPC, and sends to the destination switching apparatus a new IAM containing the VPCI of the newly selected VPC and the data negotiating information. The destination switching apparatus processes the new IAM, and sends, if the IAM is acceptable, an IAA to the originating switching apparatus. Then, after some messages are transmitted between the switching apparatuses and the terminals, the requested call is established.
Thus, in the ATM network, each switching apparatus receiving the IAM calculates a bandwidth value to be reserved on the basis of the bandwidth negotiation data, and judges whether or not the IAM is acceptable by comparing the calculated bandwidth value with the free bandwidth value. That is, DSS2 and B-ISUP allow the ATM network to contain switching apparatuses having different bandwidth calculation algorithms, and hence, used are a variety of switching apparatus having different bandwidth calculation algorithms in the ATM network.
Specifically, services offered by the ATM network can be classified into constant bit rate (CBR) service, variable bit rate (VBR) service, available bit rate (ABR) service, and unspecified bit rate (UBR) service.
Among these services, the UBR service offers no guarantee at all. (This does not imply, of course, that the most part of the data will be discarded.) Therefore, it is unnecessary to reserve bandwidth for a UBR connection. However, known are a switching apparatus which accepts a UBR connection request only when a bandwidth of X % (e.g. 5%) of the declared peak cell rate (PCR) can be reserved, and a switching apparatus which accepts a UBR connection request only when the number of connections which has been established is within the predetermined number.
The CBR service guarantees bandwidth and delay. Consequently, each switching apparatus is generally so constructed to reserve, when accepting a CBR connection request, the bandwidth the value of which is PCR/Y. Here, the Y is a constant value (.ltoreq.1) established on the basis of the buffer size. In the switching apparatus having relatively large buffer, "1" is used as the Y. In the switching apparatus having smaller buffer, smaller value (e.g. 0.95) is used as the Y.
Call set up operation using the DSS2 and B-ISUP allows to set up a call across the switching apparatuses having such different bandwidth calculation algorithms, however, each conventional switching apparatus is so constructed, as discussed above, as to select a VPC whose free bandwidth value is over the calculated bandwidth value and has a minimum difference from the calculated bandwidth value for establishing the call, and to send the IAM indicating the use of the selected VPC to the next switching apparatus.
Consequently, in the conventional ATM network, when a connection which needs a bandwidth nearly equal to the free bandwidth value is established, there is high a high probability that the IAM is rejected due to a difference of the bandwidth calculating algorithms. When the IAM is rejected, the switch apparatus which issued the IAM must select other VPC and re-sent the IAM, with the result that call setting takes a comparatively long time.
Note that, in order to reduce the probability that the IAM is rejected due to a difference of the bandwidth calculating algorithm, the switching apparatus may be constructed so that the VPCs are selected sequentially from those having larger free bandwidth. In the thus constructed switching apparatus, however, it follows that the respective VPCs are used in such a form as to make smaller the maximum of the free bandwidth values of the plurality of VPCs. Namely, the respective VPCs are used in such a form as to have a smaller probability that the VPC having the larger free bandwidth might be left in the ATM switching apparatus concerned than in the ATM switching apparatus wherein the VPCs are selected sequentially from those having smaller free bandwidth.